This multi-center, prospective trial is designed to determine the prevalence and consequence of non-tuberculous mycobacteria (NTM) in CF lung disease. The cross-sectional study to prospectively screen CF patients' sputum will determine the prevalence of NTM. A prospective cohort study is nested within the cross-sectional study to determine the clinical course of incident cases and matched controls. In the cross-sectional study we have enrolled 47 patients. To date, no patients have entered the prospective cohort study. Enrollment has been completed and we expect this study to end in June 1996.